The "parser" parses the MIME stream. A parser is an instance of MIME::Parser. You hand it an input stream (like a filehandle) to parse a message from: if the parse is successful, the result is an "entity".

A parsed message is represented by an "entity". An entity is an instance of MIME::Entity (a subclass of Mail::Internet). If the message had "parts" (e.g., attachments), then those parts are "entities" as well, contained inside the top-level entity. Each entity has a "head" and a "body".

The entity's "head" contains information about the message. A "head" is an instance of MIME::Head (a subclass of Mail::Header). It contains information from the message header: content type, sender, subject line, etc.

The entity's "body" knows where the message data is. You can ask to "open" this data source for reading or writing, and you will get back an "I/O handle".

You can open() a "body" and get an "I/O handle" to read/write message data. This handle is an object that is basically like an IO::Handle... it can be any class, so long as it supports a small, standard set of methods for reading from or writing to the underlying data source.

A typical multipart message containing two parts -- a textual greeting and an "attached" GIF file -- would be a tree of MIME::Entity objects, each of which would have its own MIME::Head. Like this:

You usually start by creating an instance of MIME::Parser and setting up certain parsing parameters: what directory to save extracted files to, how to name the files, etc.

You then give that instance a readable filehandle on which waits a MIME message. If all goes well, you will get back a MIME::Entity object (a subclass of Mail::Internet), which consists of...

A MIME::Head (a subclass of Mail::Header) which holds the MIME header data.

A MIME::Body, which is a object that knows where the body data is. You ask this object to "open" itself for reading, and it will hand you back an "I/O handle" for reading the data: this could be of any class, so long as it conforms to a subset of the IO::Handle interface.

If the original message was a multipart document, the MIME::Entity object will have a non-empty list of "parts", each of which is in turn a MIME::Entity (which might also be a multipart entity, etc, etc...).

Internally, the parser (in MIME::Parser) asks for instances of MIME::Decoder whenever it needs to decode an encoded file. MIME::Decoder has a mapping from supported encodings (e.g., 'base64') to classes whose instances can decode them. You can add to this mapping to try out new/experiment encodings. You can also use MIME::Decoder by itself.

All message composition is done via the MIME::Entity class. For single-part messages, you can use the MIME::Entity/build constructor to create MIME entities very easily.

For multipart messages, you can start by creating a top-level multipart entity with MIME::Entity::build(), and then use the similar MIME::Entity::attach() method to attach parts to that message. Please note: what most people think of as "a text message with an attached GIF file" is really a multipart message with 2 parts: the first being the text message, and the second being the GIF file.

When building MIME a entity, you'll have to provide two very important pieces of information: the content type and the content transfer encoding. The type is usually easy, as it is directly determined by the file format; e.g., an HTML file is text/html. The encoding, however, is trickier... for example, some HTML files are 7bit-compliant, but others might have very long lines and would need to be sent quoted-printable for reliability.

See the section on encoding/decoding for more details, as well as "A MIME PRIMER" below.

Which encoding you choose for a given document depends largely on (1) what you know about the document's contents (text vs binary), and (2) whether you need the resulting message to have a reliable encoding for 7-bit Internet email transport.

In general, only quoted-printable and base64 guarantee reliable transport of all data; the other three "no-encoding" encodings simply pass the data through, and are only reliable if that data is 7bit ASCII with under 1000 characters per line, and has no conflicts with the multipart boundaries.

I've considered making it so that the content-type and encoding can be automatically inferred from the file's path, but that seems to be asking for trouble... or at least, for Mail::Cap...

MIME-tools is a large and complex toolkit which tries to deal with a wide variety of external input. It's sometimes helpful to see what's really going on behind the scenes. There are several kinds of messages logged by the toolkit itself:

Usage messages are currently only logged if $^W is set true and MIME::Tools is not configured to be "quiet".

When a MIME::Parser (or one of its internal helper classes) wants to report a message, it generally does so by recording the message to the MIME::Parser::Results object immediately before invoking the appropriate function above. That means each parsing run has its own trace-log which can be examined for problems.

For example, if your mail-handling code absolutely must not die, then perform mail parsing like this:

$entity = eval { $parser->parse(\*INPUT) };

Parsing is a complex process, and some components may throw exceptions if seriously-bad things happen. Since "seriously-bad" is in the eye of the beholder, you're better off catching possible exceptions instead of asking me to propagate undef up the stack. Use of exceptions in reusable modules is one of those religious issues we're never all going to agree upon; thankfully, that's what eval{} is good for.

As of 5.3xx, the parser tries extremely hard to give you a MIME::Entity. If there were any problems, it logs warnings/errors to the underlying "results" object (see MIME::Parser::Results). Look at that object after each parse. Print out the warnings and errors, especially if messages don't parse the way you thought they would.

The MIME standard allows for text strings in headers to contain characters from any character set, by using special sequences which look like this:

=?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?=

To be consistent with the existing Mail::Field classes, MIME::Tools does not automatically unencode these strings, since doing so would lose the character-set information and interfere with the parsing of fields (see "decode_headers" in MIME::Parser for a full explanation). That means you should be prepared to deal with these encoded strings.

The most common question then is, how do I decode these encoded strings? The answer depends on what you want to decode them to: ASCII, Latin1, UTF-8, etc. Be aware that your "target" representation may not support all possible character sets you might encounter; for example, Latin1 (ISO-8859-1) has no way of representing Big5 (Chinese) characters. A common practice is to represent "untranslateable" characters as "?"s, or to ignore them completely.

To unencode the strings into some of the more-popular Western byte representations (e.g., Latin1, Latin2, etc.), you can use the decoders in MIME::WordDecoder (see MIME::WordDecoder). The simplest way is by using unmime(), a function wrapped around your "default" decoder, as follows:

One place this is done automatically is in extracting the recommended filename for a part while parsing. That's why you should start by setting up the best "default" decoder if the default target of Latin1 isn't to your liking.

RFC 2045 dictates that MIME streams have lines terminated by CRLF ("\r\n"). However, it is extremely likely that folks will want to parse MIME streams where each line ends in the local newline character "\n" instead.

An attempt has been made to allow the parser to handle both CRLF and newline-terminated input.

The "7bit" and "8bit" decoders will decode both a "\n" and a "\r\n" end-of-line sequence into a "\n".

The "binary" decoder (default if no encoding specified) still outputs stuff verbatim... so a MIME message with CRLFs and no explicit encoding will be output as a text file that, on many systems, will have an annoying ^M at the end of each line... but this is as it should be.

TODO FIXME All encoders currently output the end-of-line sequence as a "\n", with the assumption that the local mail agent will perform the conversion from newline to CRLF when sending the mail. However, there probably should be an option to output CRLF as per RFC 2045

Let's get something straight: this is an evil, EVIL practice. If your mailer creates multipart boundary strings that contain newlines, give it two weeks notice and find another one. If your mail robot receives MIME mail like this, regard it as syntactically incorrect, which it is.

People like to hand the parser raw messages straight from POP3 or from a mailbox. There is often predictable non-header information in front of the real headers; e.g., the initial "From" line in the following message:

Please note that there is currently an ambiguity in the way preambles are parsed in. The following message fragments both are regarded as having an empty preamble (where \n indicates a newline character):

In both cases, the first completely-empty line (after the "Subject") marks the end of the header.

But we should clearly ignore the second empty line in message #2, since it fills the role of "the newline which is only there to make sure that the boundary is at the beginning of a line". Such newlines are never part of the content preceding the boundary; thus, there is no preamble "content" in message #2.

However, it seems clear that message #1 also has no preamble "content", and is in fact merely a compact representation of an empty preamble.

Why not do everything in core? Although the amount of core available on even a modest home system continues to grow, the size of attachments continues to grow with it. I wanted to make sure that even users with small systems could deal with decoding multi-megabyte sounds and movie files. That means not being core-bound.

As of the released 5.3xx, MIME::Parser gets by with only one temp file open per parser. This temp file provides a sort of infinite scratch space for dealing with the current message part. It's fast and lightweight, but you should know about it anyway.

Achim Bohnet once pointed out that MIME headers do nothing more than store a collection of attributes, and thus could be represented as objects which don't inherit from Mail::Header.

I agree in principle, but RFC 2045 says otherwise. RFC 2045 [MIME] headers are a syntactic subset of RFC-822 [email] headers. Perhaps a better name for these modules would have been RFC1521:: instead of MIME::, but we're a little beyond that stage now.

When I originally wrote these modules for the CPAN, I agonized for a long time about whether or not they really should subclass from Mail::Internet (then at version 1.17). Thanks to Graham Barr, who graciously evolved MailTools 1.06 to be more MIME-friendly, unification was achieved at MIME-tools release 2.0. The benefits in reuse alone have been substantial.

Here are some definitions adapted from RFC 1521 (predecessor of the current RFC 204[56789] defining MIME) explaining the terminology we use; each is accompanied by the equivalent in MIME:: module terms...

An "attachment" is common slang for any part of a multipart message -- except, perhaps, for the first part, which normally carries a user message describing the attachments that follow (e.g.: "Hey dude, here's that GIF file I promised you.").

In our system, an attachment is just a MIME::Entity under the top-level entity, probably one of its parts.

The "body" of an entity is that portion of the entity which follows the header and which contains the real message content. For example, if your MIME message has a GIF file attachment, then the body of that attachment is the base64-encoded GIF file itself.

A body is represented by an instance of MIME::Body. You get the body of an entity by sending it a bodyhandle() message.

This is the top portion of the MIME message, which contains the "Content-type", "Content-transfer-encoding", etc. Every MIME entity has a header, represented by an instance of MIME::Head. You get the header of an entity by sending it a head() message.

A "message" generally means the complete (or "top-level") message being transferred on a network.

There currently is no explicit package for "messages"; under MIME::, messages are streams of data which may be read in from files or filehandles. You can think of the MIME::Entity returned by the MIME::Parser as representing the full message.

No encoding is done at all. This label simply asserts that the message might contain 8-bit characters, and that lines may exceed 1000 characters in length. Such messages are the least likely to get through mail gateways.

A standard encoding, which maps arbitrary binary data to the 7bit domain. Like "uuencode", but very well-defined. This is how you should send essentially binary information (tar files, GIFs, JPEGs, etc.).

A standard encoding, which maps arbitrary line-oriented data to the 7bit domain. Useful for encoding messages which are textual in nature, yet which contain non-ASCII characters (e.g., Latin-1, Latin-2, or any other 8-bit alphabet).